In an image reading device used in a digital multifunction peripheral, in order to read an image for one line extending in a main scanning direction, linear light extending in the main scanning direction is irradiated from a light source to an original document and reflected light from the original document is condensed by a lens and made incident on an image sensor.
When the light of the light source is caused to reach the original document, in some case, a prism formed by a polyhedron is used. The prism can be formed by injection molding. When the injection molding is performed, a machining trace (a cutting trace) remains in a part of the prism. If the machining trace is present on an optical path from the light source to the original document, in some case, characteristics for lighting the original document are adversely affected.